Apple is pioneering another trend, this time in how mobile users spend their money. A new study from In-Stat says the number of mobile application store users will quadruple in the next five years.

Crediting Apple and the iPhone for the emerging market, In-Stat principal analyst David Chamberlain, forecasts that more than 100 million application store compatible mobile phones will be shipping by 2014. That's not just phones from Apple, but from all manufacturers.

Apple is already expanding the reach of the App Store by allowing developers to create in-app purchases for its customers. That means that publishers can sell books to read and game titles can sell additional levels for people to play.

It will be interesting to see if RIM and Google run into the same complaints that Apple did with its developers over the transparency of the approval process. Apple has been dogged by complaints that its app approval process is veiled in secrecy and that it could hurt the platform.

Apple also said they have had 800 million downloads from the App Store since it opened last July. In all, there are 25,000 applications on the store to choose from in a variety of categories and offered for a range of prices.

The App Store is currently available in 77 countries.

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